Abstract
The rate of increase in strength of absorption bands of SO has been measured in shock-heated mixtures of sulphur dioxide and argon. Arrhenius-type plots indicate a unimolecular first step of the order d [SO]/d
t
=
k
[SO
2
] [
M
], where [SO], [SO
2
] and [
M
] are concentrations of [SO], [SO
2
] and total gas. The apparent activation energy at around 3500 °K is 56 kcal/mole. It is shown that on unimolecular reaction theory, if four harmonic modes of oscillation in the SO
2
molecules contribute to the energy available for transformation, the true activation energy is 74 kcal/mole. This agrees with the energy of excitation to a known triplet state of SO
2
, and on this basis it is suggested that the first steps in the decomposition are SO
2
+ M = SO*
2
+
M
— 73.6 kcal/mole (1) and SO*
2
+ SO
2
= SO
3
+ SO + 25.6 kcal/mole. (2) Step (2) is spin-allowed, whereas the more direct reaction SO
2
+ SO
2
= SO
3
+ SO —48 kcal/ mole is spin-forbidden. This is an unusual type of decomposition mechanism and occurs because of the high dissociation energy of SO
2
, because the direct step of low-energy is spinforbidden, and because there is a favourably situated triplet state of the molecule.
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